Bernd Diekkrüger

Bimodal Porosity And Unsaturated Hydraulic Conductivity

Soil water characteristic curves determined on undisturbed soil samples in the laboratory were used for the calculation of the unsaturated conductivity function K(h). This computed K(h) was compared to the unsaturated conductivity data measured directly in the field. When the soil water characteristic curve was smoothed according to the van Genuchten procedure and K(h) was evaluated according to Mualems approach, the coincidence of results of both procedures was poor. When a cubic spline was used for the water retention data, the derived pore size distribution curve showed two peaks. With the assumption on a bimodal porous system, K(h) was computed for each of the subsystem of pores. The function K(h) of the whole porous system is then composed of two parts, one valid close to saturation, the second for medium wetness of the soil. At the intersection of both components of K(h) a singularity exists. The agreement between the K(h) so computed and the directly measured K data has been improved. The hypothesis on the existence of the bimodal porosity within the range of the capillary soil porous system has been therefore confirmed together with the proof on the practical importance of both subsystems when they are detected in the soil.

Evaluating spatial and temporal variability in soil erosion risk—rainfall erosivity and soil loss ratios in Andalusia, Spain

Abstract Erosion modeling techniques provide a framework for the evaluation of agricultural impacts on soil and water resources. Agricultural policies or economic incentives generally induce land use changes or even agricultural crop rotation changes. This results in a different erosional behavior of cultivated soil. Besides economic benefits, the sustainability of the agricultural practices with regard to soil and water resources has to be evaluated for specific local or regional conditions. This paper analyzes the spatial and temporal variability in soil erosion risks in a changing Mediterranean agro-ecological situation. At first the spatio-temporal variability of rainfall erosivity is analyzed. The depth and erosivity of design storms are determined for different return periods. Then, the temporal variability of soil loss ratios (SLR) due to different agricultural crop rotations are evaluated on watershed scale using the revised universal soil loss equation (RUSLE). The 211 km 2 Guadalteba river basin and study area is located in the region of Andalusia, southern Spain, presenting a typical south European agricultural wheat and oil producing area with marked occurrence of soil erosion problems. The spatial soil erosion risk evaluation approach is based on commonly available data and a minimum of additional field observations. The spatial distribution of input and output data is handled with the Integrated Land and Water Information System (ILWIS).

Scaling input data by GIS for hydrological modelling

An analysis of scaling effects is performed to evaluate whether data aggregation is a useful regionalization tool or whether it leads to an unacceptable loss of information. One issue concerns the appropriate resolution of digital elevation models (DEMs) used to derive geomorphological input parameters for hydrological models. In particular, the scale problem of watershed division by a channel network and smaller sub-basins is addressed. The investigation involved commercially available data sets with different horizontal and vertical resolutions and systematically aggregated DEMs. A stream network and the contributing subareas were derived from a DEM with a distinct critical support area. By varying this threshold area various watershed configurations were obtained. The sensitivity of surface runoff simulations to all watershed configurations was studied with synthetic storms and by means of an infiltration excess runoff model. The study revealed that elevation data with different resolutions diverge enormously in landscape representation and in the derived parameters such as slopes, flow directions and channel networks. Coarse DEMs show a smoother terrain and shorter flow paths than highly resolved data. The contributing threshold area controls the extent of the watershed configuration and therefore determines the drainage density. These topographic and geomorphological features were used to explain differences in the runoff simulation results. When watershed configurations with a varying extent of the channel network were derived from a distinct DEM and then used to simulate surface runoff, the drainage densities of the configurations correlated with the simulated runoff volume. A distinct drainage density, however, did not necessarily lead to similar simulation results when different DEMs were used. Since the hydrological model permits reinfiltration, the runoff volume depends directly on the lengths of the overland flow. Therefore, the mean length of the overland flow paths might to a certain degree be considered as a scaling factor. Copyright

Simulation of water fluxes using different methods for estimating soil parameters

Abstract The model system SIMULAT was applied to compute the water dynamics of a loam site from the Krummbach catchment and a sand site from the Eisenbach catchment, two research catchments in north Germany. The simulation period was three years. In this study two procedures of obtaining soil hydraulic properties were tested with regard to the applicability in simulation models. Simulations using measured retention curves are compared with results where the retention curves were estimated by a pedo-transfer function. Because measured retention data suggest a bimodal pore size distribution at the loam site which can not be described using standard approaches, additional simulations were carried out using bimodal retention curves.

Monitoring and Modeling the Terrestrial System from Pores to Catchments: The Transregional Collaborative Research Center on Patterns in the Soil–Vegetation–Atmosphere System

AbstractMost activities of humankind take place in the transition zone between four compartments of the terrestrial system: the unconfined aquifer, including the unsaturated zone; surface water; vegetation; and atmosphere. The mass, momentum, and heat energy fluxes between these compartments drive their mutual state evolution. Improved understanding of the processes that drive these fluxes is important for climate projections, weather prediction, flood forecasting, water and soil resources management, agriculture, and water quality control. The different transport mechanisms and flow rates within the compartments result in complex patterns on different temporal and spatial scales that make predictions of the terrestrial system challenging for scientists and policy makers. The Transregional Collaborative Research Centre 32 (TR32) was formed in 2007 to integrate monitoring with modeling and data assimilation in order to develop a holistic view of the terrestrial system. TR32 is a long-term research program ...

Modelling the spatial variability of soil moisture in a micro-scale catchment and comparison with field data using geostatistics

Abstract The temporal and spatial variability of soil moisture in the micro-scale Berrensiefen catchment is simulated with SWMS_3D (three-dimensional variably saturated flow based on finite elements) and a loosely coupled model for the atmospheric boundary conditions. An additional model component for macropore and surface runoff and their concentration time is introduced. Time step of precipitation, atmospheric parameters and the simulated hydrologic processes is one hour. Soil parameters are discretized with a horizontal resolution of 10 m and five nodal layers in the vertical direction. A high resolution digital elevation model is used to reconstruct catchment surface morphology. For a nearly 2-month period hydrologic processes are simulated. The process model results are compared with measurements and the influence of the topography, the distributed soil and land use parameters on the soil water fluxes is examined. The spatial structure of soil moisture during a drying period is analysed using geostatistics. The application of standard geostatistical techniques (variogram modelling) captures the main features of spatial variability of the modelled soil moisture distributions. The integral scale or correlation length of soil moisture is constant while the semivariance shows clear correlation with soil moisture status during that period. A geostatistical comparison of the modelled soil moisture patterns of the whole catchment with a measured data set of a single hillslope of the catchment addresses the problem of differing scales of modelling and measurements. It is shown that due to the different spatial scales the results of point measurements of soil moisture can hardly be used to validate the modelled spatial structure of soil moisture.

Comparison of the performance of pesticide-leaching models on a cracking clay soil: results using the Brimstone Farm dataset

Abstract The leaching of the pesticide isoproturon from the macroporous clay soil at Brimstone Farm was modelled using four alternative models (MACRO, CRACK-NP, SIMULAT and PLM). Model results are presented for two test periods, the whole of one winter for which daily observations are available, and a short subset for which hourly data were presented. The best results are those given by MACRO with an expert user, although satisfactory results were also obtained from CRACK-NP and for the longer test period by PLM. SIMULAT was less successful in modelling the site because it did not include an adequate representation of the site hydrology, it was unable to predict the leaching of pesticide. MACRO was also used by a second modelling group who were less familiar with both the code and the site. Although the initial uncalibrated runs from this group were poor, the final calibrated results were almost as good as those derived by the ‘expert’ user. The simulations showed the difficulty of deriving adequate representations, even where relatively complete soil physical data are available. A shortcoming of the dataset provided was the lack of detailed soil moisture observations, particularly to define the initial conditions. From a well-monitored site, many observations of site hydrology (water table position, drainflow and surface flow) were available, but significantly, fewer pesticide concentrations in either the soil or the discharges were available. Models could thus be evaluated only in terms of their ability to predict the magnitude and timing of major pesticide leaching events.

Modeling pesticide dynamics of four different sites using the model system SIMULAT.

Abstract This study aimed to assess the accuracy of SIMULAT, a computer model primarily designed for predicting the fate of pesticides in soil. The evaluation was carried out by comparing simulated results on herbicide degradation with results obtained from field and lysimeters experiments. For model validation four different data sets were available. The data sets included field, lysimeter, and laboratory experiments from Germany (Weiherbach), The Netherlands (Vredepeel), Great Britain (Brimstone), and Italy (Tor Mancina). The applied herbicides and the determined soil and water parameters varied substantially among the four empirical data sets used for model evaluation. In a first step simulations were run with the model still being uncalibrated. Afterwards a calibration of hydraulic parameters was performed using measured water and bromide contents in soil (Weiherbach, Vredepeel and Brimstone) or leachate (Tor Mancina). In contrast to the hydraulic parameters, the sorption and degradation parameters were not calibrated. Simulations were run with the calibrated model and the results compared with those obtained from field measurements. The wide range of implemented boundary conditions such as lysimeter, free drainage or fluctuating groundwater table enabled applying SIMULAT to all four data sets. However, usage of parameters obtained in laboratory experiments gave no satisfiable simulation of the degradation of the herbicides. In contrast to the macroporous loam soil at Tor Mancina, water and bromide transport were accurately simulated in the loess (Weiherbach) and in the sandy soil (Vredepeel). Severe problems occur while simulating the fluctuating groundwater and drain flow in the clay soil at Brimstone.

Regionalisation concept for hydrological modelling on different scales using a physically based model: Results and evaluation

Abstract A regionalisation concept for hydrological modelling on different scales using a physically based simulation model is presented. It enables us to calculate regional water balances by simulating a limited number of representative ecotopes (=hydro-pedotopes) instead of all units using a physically based model system (soilvegetation-atmosphere-transfer-scheme). Using a cluster analysis the hydrological quantities (e.g. monthly values of actual evapotranspiration, groundwater recharge, surface runoff and interflow of all ecotopes were analysed and groups of similar behaviour defined. For each of these clusters one representative ecotope is chosen. Results of an application of this concept are presented: The water fluxes of a 1000 km2 catchment in Germany were calculated and simulated runoff compared with measured. Simulating selected representative instead of all ecotopes reduces the number of necessary simulation runs by up to 95%. The results suggest that the quality of the simulation is not effected by using representative instead of all ecotopes. Because the data were not aggregated the spatial pattern of properties and fluxes is preserved. In order to evaluate the model results, the uncertainties concerning the data base (e.g. soil map, weather data) and the assumptions included in the regionalisation concept are investigated. Because in this concept no model calibration is performed, the applicability is only limited by the data availability (spatial and temporal resolution). The model shows a reliable hydrological behaviour and is a suitable tool for landscape management.

Seasonal soil moisture patterns: Controlling transit time distributions in a forested headwater catchment

The Transit Time Distribution (TTD) of a catchment is frequently used for understanding flow paths, storage characteristics, and runoff sources. Despite previous studies, the connections between catchment characteristics and TTDs are still not fully understood. We present results from a 2 year stable isotope tracer investigation in the forested Wustebach headwater catchment (38.5 ha), including precipitation, stream, and tributary locations. We used the gauged outlet to determine effective precipitation (peff), subdivided for wet and dry catchment state, and assumed it to be spatially uniform. We then calculated TTDs of 14 ungauged stream and tributary locations where stable isotope tracer information was available and compared them to respective subcatchment areas and the proportion of riparian zone within the subcatchments. Our approach gave insight into the spatial heterogeneity of TTDs along the Wustebach River. We found that hydrological hillslope-riparian zone disconnection was an important factor, as the catchment shifted between two distinct, time-variant hydrological responses that were governed by seasonal changes of overall catchment wetness. The difference in hydrological behavior of the riparian zone and hillslopes could explain the often encountered “old water phenomenon,” where considerable amounts of old water quickly appear as runoff. TTD results showed a negative correlation between riparian zone proportion and Mean Transit Time (MTT), corroborated by the dense network of soil water content measurements. No correlation between subcatchment size and MTT was found.